An oil pan provided in a lower portion of an engine for a vehicle has an approximately rectangular bottom wall extending in a direction of each cylinder line as viewed from the upper side, as disclosed in, for example, U.S. Patent Publication No. 2009/0151690. The oil pan has a deep bottom section provided on one side of the bottom wall and a shallow bottom section provided on the other side of the bottom wall. Plural reinforcing ribs are formed on an inside of both the deep bottom section and the shallow bottom section, and plural reinforcing ribs are formed also on an outside of the shallow bottom section. In this way, the conventional oil pan structure has plural reinforcing ribs provided on the bottom wall, which can not only increase rigidity of the bottom wall, but also reduce vibration of the bottom wall.
However, the above oil pan structure may result in an increased weight of the oil pan, due to the reinforcing ribs.
The present invention is made so as to address such a problem. Therefore, a purpose of the invention is to provide an oil pan structure, wherein the rigidity of the bottom wall could be increased while reducing the weight of the oil pan.